Fish-finders and echo-sounders applying ultra-sonic sound wave are eouipped to many fishing boats for the use of fishing operation and depth finding. But, they are not gaining necessarily satisfctory effect beacause of permeation loss by ship's bottom plate when they are eouipped in fishing boats. When transducer of fish-finder is eouipped on the bottom of a boat, the tansducer is put in a tank filled with fresh water which is fixed to the bottom plate ; when the transducer is equipped on the side of boat, the transducer is put in a stream lined cover made of steel or rubber and fresh water is used as medium to prevent running attenuation and running noise. When the ultra-sonic waves transmit through steel plate or rubber film, they are attenuated remarkably. But the permeation loss caused by steel plates and othes can be lessened by means of oil medium instead of fresh water. In this experiment, botanical oil was used and basic study was made on the influences of oil upon the cnaracteristics of ultrasonic. And the experiment shows the fact, if oil is used as the medium when the ultra-sonic waves transmit through steel plate of 10mm in thickness, it is as if a steel plate of 5mm in thickness was applied. As the result of the acove study, it isconsidered that the oscillation power of transmitter can be increase by oil and that the efficiency of fish-finder and echo-sounder can be enhnced. The study is being continued now.